Pollution control devices are used on motorized vehicles to reduce atmospheric pollution. Two types of pollution control devices are currently in widespread use: catalytic converters and diesel particulate filters or traps. Catalytic converters contain one or more catalysts, which are typically coated onto a substrate in the form of a monolithic structure. The monolithic structures are usually ceramic, although metal monoliths have been used. The catalyst(s) can oxidize carbon monoxide, oxidize various hydrocarbons, reduce the oxides of nitrogen, or a combination thereof in exhaust gases. Diesel particulate filters or traps are typically in the form of wall flow filters having a honeycombed monolithic structure made from permeable crystalline ceramic materials. Alternate cells of the honeycombed monolithic structure are plugged so that the exhaust gas enters one cell, flows through the permeable wall into another cell, and then exits the structure.
In state-of-the-art constructions of these pollution control devices, the monolithic structure is enclosed within a end cone housing. Because the monolithic structure typically has a larger diameter than an exhaust pipe from a vehicle, the end cone housing typically includes a transition zone. This transition zone, referred to as the end cone region, narrows from a diameter suitable for the monolithic structure to a diameter suitable for connection to an exhaust pipe. The end cone is usually conical in shape and can be provided on both the inlet and outlet side of the pollution control device.
Pollution control devices are usually operated at a relatively high temperature such as, for example, above about 500° C. Consequently, insulation is typically provided within the end cone housing. Insulation material in the form of a mounting mat can be placed between the monolithic structure and the metal housing. Insulation can also be placed in the end cone region of the end cone housing. The end cone region typically has a double-wall construction that includes an outer end cone housing and an inner end cone housing. Insulation material can be placed between the inner and outer end cone housings.